Second set of new devices, functionality and cleanup for IIO in the 4.1 cycle.

New Functionality
* Watermark logic for buffers.  Allows for blocking reads to block until
  their requested amount is available - previously they only blocked until
  a single scan of data was available.  Also allows for polling for a
  watermark amount of data to be available.  This feature was first proposed
  some time ago to good responses, but not taken further by the original author
  Octavian has picked up the gauntlet and taken it through
  to merging (along with the hardware fifo support that follows).
* New approach to hardware fifo handling - in particular handling the
  interaction of a hardware fifo feeding into a software fifo and their
  watershed events.  We don't have every possible case well covered yet,
  but this is certainly a good, flexible starting point.  This will replace
  the previous approach used in ancient drivers (sca3000) where we just
  exposed the hardware buffer directly to userspace.  Very few pieces of
  hardware have sufficiently long buffers for that to be an adequate solution.
* bmc150_accel - hardware fifo support.
* mlx90614 - support dual IR sensor devices + some refactoring to clean up the
  code and allow some other functionality currently under review.
* L3GD20H gyroscope support added to the st_gyro driver.
* lis3lv02d accelerometer added to the st_gyro driver. Note this part is
  also supported by the older lis3 driver under misc.  A lengthy discussion
  took place and concluded that holding parts out on the basis that whole
  driver would be subsumed into this one was counter productive.  Better
  to add part support and add additional features as people need them.
  Basically there was not advantage in not merging the support.
* max517 driver gains support for MAX520 and MAX521 DACs.

Documentation
* 3.20 -> 4.0 renaming for recent docs. Whilst technically a fix, I think
  people will cope until the next merge merge window.
* An ABI typo hat -> What:  More ABIs should have hats.
* Document in_rot_offset, illuminance_raw and illuminance_scale.

Cleanups
* Fix a scale extraction bug in generic_buffer.c example.
* Constify a load of device tree related structures.
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